Ethylene/vinyl acetate (EVA) is a copolymer widely used, inter alia, in solar panels, as an encapsulating material to protect solar cells (and especially the semiconductors contained therein) from outdoor environmental elements, especially moisture and UV radiation, and to provide electrical insulation. EVA indeed provides a good transparency and adhesion to the substrates of the photovoltaic (PV) module, together with a high resistivity and good moisture-barrier effect. These properties may alternatively be used in the manufacture of laminated glass.
It is common practice to crosslink these EVA copolymers in order to improve their thermal stability, especially their creep strength, their adhesion to the substrates and their weathering degradation resistance. For this purpose, various crosslinking agents have been used to date in the EVA formulation, which are typically peroxides such as dicumyl peroxide (DCP), peroxyesters, peroxyketals, peroxycarbonates and mixtures thereof. An example of monoperoxycarbonate used for this purpose is OO-t-butyl-O-2-ethylhexyl-monoperoxycarbonate (TBEC). This peroxide has proven to require a lower operating temperature than DCP, without providing the resulting product with a yellowish colour. It is thus used in the manufacture of photovoltaic modules (see for instance K. Thaworn et al., Open Journal of Polymer Chemistry, 2012, 2, 77-85). The Applicant has further shown that the addition of OO-t-amyl-O-2-ethylhexyl-monoperoxycarbonate (TAEC) to TBEC shortened the reaction time and improved the crosslinking density of EVA, which resulted in an increased tensile strength and a high modulus of the products made from the crosslinked EVA (WO 2010/007315).
During the laminating process of the PV module, the EVA composition is first deposited onto the frontsheet, then covered by the solar cells and deposited again thereon, before applying the backsheet, so as to obtain a PV module which is then heated at a certain high temperature for some time and pressed into place, whereby the EVA composition is cured.
It has been found that the EVA composition was susceptible to premature crosslinking in the barrel or die head of the extruder in which it is processed, prior to the formation of the above laminate. This phenomenon, which is called “scorching”, results in irregularities in the EVA sheet thus formed, which in turn impairs the appearance and properties of the PV module. In some cases, pressure may also build up in the extruder, which requires discontinuing the extrusion process. This has especially been observed in the case of EVA having a relatively low melt-flow index and/or a relatively narrow molecular weight distribution (source: U.S. Pat. No. 4,015,058). However, on the other hand, in order to achieve commercially feasible process speeds, it is necessary that once the EVA sheets have been shaped and then heated above the thermal decomposition temperature of the peroxides, crosslinking proceeds as rapidly as possible in order to increase the economics of the process and also minimize possible side reactions.
Various solutions have been proposed to date to prevent scorching of EVA compositions. For instance, it has been suggested to add polymerization inhibitors to the EVA composition. However, unwanted yellowing has been noted. Alternatively, it has been proposed in U.S. Pat. No. 4,015,058 to add at least 1 wt. % of cumene hydroperoxide and/or tertiary butyl hydroperoxide to dicumyl peroxide (DCP). However, the crosslinking rate obtained with this system is not high enough for industrial applications, in particular in the manufacture of PV modules, due to the presence of DCP. Because of the aromatic structure of DCP, the yellowing problems also still remain.
Another solution has been provided in JP2011-140588, which is said to be appropriate for the manufacture of PV modules. It consists in adding from 4 to 50 parts by weight of a hydroperoxide, such as t-butyl hydroperoxide, to 100 parts by weight of an organic peroxide selected from a monoperoxycarbonate, a dialkyl peroxide, a peroxyketal and a peroxyester. Although this solution allows overcoming the drawbacks associated with the use of DCP, it has been found in JP2011-140588 that the above amounts of hydroperoxide negatively affected EVA crosslinking density. In this respect, it was suggested in this document to increase the total amount of monoperoxycarbonate and hydroperoxide while keeping the ratio of hydroperoxide to monoperoxycarbonate constant, i.e. around 20% (see Table 1).